The present invention relates to devices for measuring electrical currents, and more particularly to integrating electrometers for measuring fluctuating currents. The invention further relates to instruments that use integrating electrometers to measure electrical currents to generate particle concentration distributions based on electrical mobility.
Since the late 1960s, those skilled in the art have relied on electrical mobility in a variety of instruments for counting and sizing the particles in aerosols, i.e. suspensions of particles in gaseous media. One such instrument is shown schematically in FIG. 1. An aerosol including a polydisperse particulate suspension is introduced into a cylindrical housing 1 at an entrance 2 where an electrical charge is applied to the particles. A sheath flow of filtered air is introduced to the housing through a conduit 3. A cylindrical center electrode 4 is maintained at a high voltage to attract the charged particles, while an outer wall 5 of the housing is maintained at ground. As the aerosol and sheath flows merge and proceed downward, the charged particles move inwardly toward electrode 4 at different rates that depend on their electrical mobility. Particles with higher mobility, typically the smaller particles, impinge upon electrode 4 above a slot 6. Particles with lower mobilities impinge upon the electrode below the slot. Particles within a desired electrical mobility range enter the slot and travel out through a conduit 7 as a monodisperse aerosol. The remainder of the original aerosol, and sheath flow, exit the instrument through an exhaust conduit 8.
By stepping the voltage applied to electrode 4 through a series of different levels, and separately counting particles exhibiting the desired electrical mobility at each level, this instrument can be used to generate a size distribution or spectrum of the particles. This approach is satisfactory when the particle distribution in the aerosol is expected to remain fairly stable, e.g. in environmental monitoring. However, in situations where the particulate makeup of the aerosol is likely to experience rapid fluctuations, such as in studies of engine exhaust particles, the time required for stepping the electrode voltage through the desired series of levels can interfere with the need to track rapidly changing conditions. Further, this approach cannot be used to generate discreet measurements representing different sizes of particles within the same packet or volume of the aerosol as it progresses through the instrument.
To address these concerns, particle measuring instruments have been developed with modified electrode configurations to enable simultaneous generation of multiple currents for particle sizing. In particular, one recently developed instrument features a single central column electrode biased to one or more positive voltage levels, surrounded by an axially extending series of annular collector electrodes maintained at ground. Smaller, higher-mobility particles are collected at the upstream electrodes, while larger, lower-mobility particles are collected at the downstream electrodes. Each electrode generates its own measurement current, based on the collection of particles within its range of electrical mobility, which is used to represent a particle size range. Accordingly, multiple size measurements are taken simultaneously, with no need to step through multiple levels of biasing voltage. Further, individual volumes or packets of the aerosol can be measured in multiple channels or stages, each corresponding to one of the collector electrodes, as the aerosol volume proceeds through the instrument.
Electrical currents generated by particle collection at the annular electrodes are measured by electrometer amplifiers, each generating an output voltage representative of the incoming current. Traditionally, the electrometer amplifier circuits have employed operational amplifiers with low input bias currents and high value resistors in their feedback loops. Currents to be measured are provided to the negative input terminals of the operational amplifiers. As a result, the output voltage U is generated according to the formula U=−IR, where I is the incoming current and R is the resistance.
While this arrangement is better suited to track changing particulate levels in monitored aerosols, thermal noise due to the high value feedback resistor requires use of a low pass filter to remove the noise component. This limits the bandwidth of the measuring circuit, reducing its ability to accurately follow fluctuations in particle sizes and concentrations. Accordingly, this approach is not entirely satisfactory for following the transients inherent in engine exhausts.
Accordingly, it is an object of the present invention to provide an instrument for characterizing aerosol particles based on their electrical mobility, with the capacity to accurately classify particles by size, based on low amplitude particle collection currents subject to rapid fluctuations.
Another object is to provide an aerosol particle spectrometer capable of generating, simultaneously, multiple currents representative of particles in different ranges of electrical mobility, sampled at a high rate to achieve improved tracking of fluctuations in the multiple currents.
A further object is to provide an integrating electrometer circuit configured to generate an output that rapidly and accurately characterizes the incoming current, to provide more reliable tracking of rapidly fluctuating incoming currents.
Yet another object is to provide a process for measuring an electrical current by integrating the current over a range predetermined by at least one threshold signal level, using the threshold level to rapidly reset an output based on the incoming current by positively driving the output back into the operating range for further integration.